1. Field of the Invention
The present invention relates to an antenna system for use in a mobile radio communication terminal such as a cellular telephone or a pager, and in particular to a polarization diversity antenna system used therefor.
2. Description of Related Art
Space diversity antenna schemes have been employed in radio communication systems e.g., mobile radio communication systems, to improve the reception capability of a receiver under conditions of fading. As is well-known, the space diversity scheme employs a plurality of antennas. A typical space diversity antenna system for a cellular telephone, for example, incorporates a whip antenna protruding from the body of the cellular telephone and a built-in antenna for space diversity. Since the base station managing the cellular telephone communicates therewith using vertical polarized waves, the two antennas are so positioned as to be capable of receiving vertical polarized waves. In particular, the built-in antenna is located in the upper portion of the body so as to receive the waves without interference by the hand of the user of the cellular telephone.
FIG. 1 schematically shows a conventional space diversity antenna system. The antenna is used in the 800 MHz band, for example. In FIG. 1, the space diversity antenna system incorporates a circuit board 10, a whip antenna 20, and a built-in antenna 30. The circuit board 10, on which integrated circuits and electric parts are assembled, is placed in the body of the cellular telephone. The whip antenna 20 is provided on the upper side of the body in the vertical direction, that is, in the Z direction. The built-in antenna 30 is positioned in the upper portion of the circuit board 10. The shape of the built-in antenna 30 is like inverted F so as to save space for installation, wherein the built-in antenna 30 is shown magnified in the dotted circle in the figure. Here, for example, the length of the whip antenna is 90 mm, the height of the body is 120 mm, and the width of the body is 35 mm, the width depending upon the wavelength of the radio wave. Further, the height, width, and depth of built-in antenna 30 are 5 mm, 35 mm, and 20 mm, respectively.
FIG. 2 shows the characteristics of the conventional space diversity antenna system of FIG. 1, wherein FIG. 2(A) shows the radiation pattern of the built-in antenna 30 and FIG. 2(B) shows the radiation pattern of the whip antenna 20 in the Y-Z area of FIG. 1. As shown in FIG. 2, both the built-in antenna 30 and the whip antenna 20 have such characteristics that the component .theta. strongly radiates horizontally, namely in the Y direction, while the component .phi. weakly radiates horizontally. Here, since transmission antenna systems and reception antenna systems are reversible, "to radiate" is equivalent to "to receive". To summarize, in FIG. 2, both the built-in antenna 30 and the whip antenna 20 have a strong directionality for a vertical polarized wave in the Y direction.
In the conventional space diversity antenna system, however, since the built-in antenna 30 is adjacent to the whip antenna 20, the two antennas fails to sufficiently yield the effect of space diversity. Two antennas so located also cause each other's characteristics to deteriorate due to the electromagnetic combination of the two antennas.
On the other hand, in addition to the space diversity system, the polarization diversity system is known. The polarization diversity system requires the built-in antenna 30 to receive polarized waves differing in directionality from the polarized waves received by the whip antenna 2, that is, to receive horizontal polarized waves. However, since the small body of the cellular telephone does not permit a large circuit board or a large ground plate for grounding, a large current is produced in the ground plate by the vertical radiated waves received by the ground plate. Accordingly, the characteristic of the built-in antenna 30 depends on a high-frequency current vertically flowing in the ground plate more than on the high-frequency current flowing in the built-in antenna 30 itself. In sum, the effect is that the current flowing in the ground plate prevents the built-in antenna 30 from being able to receive horizontal radiation.
FIG. 3 schematically shows a model of a polarization diversity antenna system with the built-in antenna placed horizontally, and FIG. 4 shows the characteristic of the built-in antenna. Although the built-in antenna 30 is placed horizontally, as shown in FIG. 4, the component .theta. strongly radiates horizontally, in approximately the same fashion as shown in the characteristic shown in FIG. 2(A). The reason why the direction of radiation of the built-in antenna 30 remains unchanged is that the built-in antenna 30 depends mainly on the high-frequency current vertically flowing in the circuit board 10. Consequently, the polarization diversity antenna system constructed as in FIG. 3 fails to provide the effect of polarization diversity.